1. Field of the Invention
The present invention generally relates to free-point indicator apparatus for locating the point at which a pipe is stuck in a borehole, and more particularly to an improved free-point tool operating on direct current and employing only resistance measurements.
2. Description of the Prior Art
There are a number of applications such as, for example, in the drilling of wells or the like when a pipe becomes stuck in a borehole and it becomes desirable to ascertain the exact location in the borehole where the pipe is stuck. Measuring instruments known as free-point tools or indicators are in common use for this purpose. One such free-point indicator is disclosed in U.S. Pat. No. 3,004,427 which employs a variable inductance supplied with an alternating current. The variable inductance has a magnetic core divided into two portions which are separated by movement of the pipe when tension is applied, resulting in a change in inductance which can be measured at the surface. Other such free-point indicators use oscillators and determine by a charge of frequency whether or not a pipe is free or stuck.
While the prior art free-point indicators have been used with varying degrees of success in the past, the accuracy with which these instruments measure is often affected by the type, weight and magnetic properties of the metal of the pipe and, in some cases, by the earth's magnetic field. Furthermore, the prior art free-point indicators are typically rather complex devices requiring multiconductor electrical cables and expensive electronic circuitry.